Methods for recovering natural gas using nitrogen rejection units

ABSTRACT

A method and system for enhanced oil recovery by performing the steps of feeding a mixture of nitrogen from a primary nitrogen supply and an optional supplemental nitrogen supply into an oil field; separating recovered oil from a gas mixture comprising nitrogen, natural gas and C2+ hydrocarbons; feeding the gas mixture to a nitrogen rejection unit operating at elevated pressures; and recovering the nitrogen, natural gas and C2+ hydrocarbons. A method for the recovery of natural gas is also described herein.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. provisionalapplication Ser. No. 62/028,091, filed on Jul. 23, 2014.

BACKGROUND OF THE INVENTION

The present invention relates to the use of high pressure nitrogenrejection units along with supplemental air separation units in enhancedoil recovery operations to improve performance.

Nitrogen injection in mature oil fields maintains the field pressure andassists in increasing oil production. The nitrogen content of an oilfield will increase depending upon its age and maturity. Nitrogenrejection units (NRUs) are used to separate nitrogen from natural gasthereby resulting in a saleable natural gas, The nitrogen that isseparated is either vented or recompressed and fed back into the oilfield for use in enhanced oil recovery operations.

Conventional NRUs are designed for nitrogen contents of approximately20% in the feed gas (see for example, Arthur J. Kidnay, William R.Parrish, Fundamentals of Natural Gas Processing, 2006). Those NRUsoperate at low pressures to provide saleable natural gas at economicoperating conditions. The produced natural gas and the nitrogen willrequire significant compression power downstream of the NRU to feed thenatural gas into a pipeline and the nitrogen back for enhanced oilrecovery (FOR) operations, respectively. Some of the produced naturalgas is also used to provide the required compression power.

However, mature oil fields may reach a nitrogen concentration that isgreater than 80% nitrogen. This high nitrogen concentration makes theoperation of the originally installed NRUs uneconomical or almostimpossible to produce saleable natural gas. Oil plant owners may facethe choice that it is more economic to bypass the IRU and recompress thenitrogen rich gas which still contains up to 20% hydrocarbons for EOR.In this instance the oil plant is not producing any more natural gas. Toprovide the required compression power for the EOR operations, the oilplant owner would need to actually buy natural gas to drive thecompressors which increases their operation expenses significantly.

The present invention overcomes these difficulties by using a primarysource of nitrogen and a supplemental source of nitrogen to assist inrecovering oil and a gas mixture from the oil field. While the oil isrecovered, a nitrogen rejection unit is then used to separate the gasmixture into component portions which can be utilized for example as afuel source for the compressors used in the nitrogen feeding operation.

SUMMARY OF THE INVENTION

In a first embodiment of the invention, there is disclosed a method forenhanced oil recovery comprising the steps of (a) feeding nitrogen froma primary nitrogen supply into an oil field; (b) recovering oil and agas mixture comprising nitrogen, natural gas and C2+ hydrocarbons fromthe oil field; separating the oil from the gas mixture (d) feeding thegas mixture to a nitrogen rejection unit; and (a) recovering nitrogen,natural gas and C2+ hydrocarbons.

The method may further comprise feeding a supplemental nitrogen supplyinto the oil field.

The primary nitrogen supply is from an air separation unit and thesupplemental nitrogen supply is from a supplemental air separation unit.The primary nitrogen supply is fed to a high pressure compressor whichpreferably operates at a pressure up to 6700 pounds per square inch,

The oil and the gas mixture are fed to an oil separation and sour gasremoval unit, thereby separating the recovered oil from the gas mixture.

The recovered oil may be addressed in a manner of way but is typicallyfed to a storage facility.

The nitrogen rejection unit will separate the nitrogen from the naturalgas and C2+ hydrocarbons. The nitrogen rejection unit will typicallyoperate at elevated pressures of about 30 bar.

The recovered nitrogen is fed to an intermediate pressure compressorbefore being fed to the high pressure compressor.

A mixture of nitrogen and methane is recovered from said nitrogenrejection unit and is fed to the high pressure compressor.

Alternatively, the C2+ hydrocarbons may be fed along with the nitrogenand methane to the high pressure compressor.

The recovered C2+ hydrocarbons may be fed to a recycle or fractioningunit.

In a second embodiment of the invention, there is disclosed a method forthe recovery of natural gas from an enhanced oil recovery operationcomprising the steps of (a) feeding a primary nitrogen supply into anoil field; (b) recovering oil and a gas mixture comprising nitrogen,natural gas and C2+ hydrocarbons from the oil field; (c) separating theoil from the gas mixture; (d) feeding the gas mixture to a nitrogenrejection unit operating at elevated pressures; and (e) separatingnatural gas from nitrogen and C2+ hydrocarbons.

In a third embodiment of the there invention, there is disclosed asystem for enhanced oil recovery comprising a source of nitrogen, asupplemental source of nitrogen, and a nitrogen rejection unit.

The source of nitrogen is a primary air separation unit. Thesupplemental source of nitrogen is a supplemental air separation unit.

The nitrogen rejection unit operates at high pressure of 30 bar.

The nitrogen rejection unit separates a gas mixture or nitrogen, naturalgas and C2+ hydrocarbons.

The system further comprises an intermediate pressure compressor.

The system further comprises a high pressure compressor.

About 50% of the hydrocarbon rich stream recovered can be used formixing with the existing fuel gas stream. This modification of the fuelgas stream enables adjustment of its energy content and the specificheating value to the specific compressor requirements for bothintermediate and high pressure compressors. The remainder of thehydrocarbon rich stream is valuable due to its energy content andrelatively high concentration of C2+ components and may be furtherpurified to produce a saleable product.

The advantages achieved by the present invention include a reduction inoverall operating expenses by reducing the amount of natural gaspurchased as well as maintaining the recycle stream nitrogen pressure.Due to the separation of the nitrogen and hydrocarbons in the nitrogenrejection unit, the content and flow of recompressed hydrocarbons forenhanced oil recovery is significantly reduced. There is also lesscompression power required to recompress the nitrogen stream from thenitrogen rejection unit outlet pressure of approximately 30 bar to theenhanced oil recovery pressures of approximately 6800 psi compared toconventional low pressure nitrogen rejection units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the method for nitrogen rejection according tothe invention.

FIG. 2 is a schematic of a nitrogen rejection unit according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic of an enhanced oil recovery operation. An airseparation unit (ASU) of 32 million standard cubic feed per day (MMSCFD)A provides nitrogen through line 1 to line 4. This nitrogen is fedthrough line 4 to a high pressure compressor B which is capable ofcompressing the nitrogen to 6700 pounds per square inch (PSI).

This high pressure nitrogen stream is fed through line 6 to the oilfield E. A supplemental ASU D operating at 17.8 MMSCFD also producesnitrogen and fees this supplemental nitrogen into line 6 through line 5.The combined nitrogen streams are at 96% nitrogen purity and 103 MMSCFDwhen fed into the oil field E.

The enhanced oil recovery operations are undertaken at the oil fieldformation E and the output from the enhanced oil recovery operation isan oil and gas mixture which is fed through line 8 to an oil separationprocess, sour gas removal and chiller and deethanizer F. The gas that isrecovered from this operation is primarily a mixture of nitrogen,natural gas and C2+ which is fed through line 9 to a nitrogen rejectionunit (NRU) G operating at elevated pressures of approximately 30 bar.The nitrogen rejection unit will produce a 96% purity nitrogen feed atquantities of 53 MMSCFD and feed this through line 10 to an intermediatepressure compressor unit C.

The intermediate pressure compressor C receives the original feed fromthe existing ASU A through line 3 and an additional feed from the NRU Gthrough line 10. The intermediate pressure compressor C will combinethese two feed and will pressurize the nitrogen to 2250 psi. Thispressurized combination can be fed through line 4 to the high pressurecompressor B for ultimate entry into the oil field E.

The nitrogen rejection unit G operating at elevated pressures willfurther produce a stream of natural gas and nitrogen which is recoveredthrough line 12 at a feed rate of 4.5 MMSCFD as well as a stream ofgreater than 85% purity C2+ hydrocarbons at a feed rate of 4.8 MMSCFDwhich is recovered through line 11.

This C2+ hydrocarbon stream is fed through line 11 to line 13 where itwill be fed into a C2+ recycle or fractioning unit H. Alternatively, theC2+ hydrocarbons that are recovered can be fed through line 14 to thehigh pressure compressor B. This stream will be joined by the mixture ofnatural gas and nitrogen recovered through line 12 which can also be fedto line 14. The resulting mixture as fed through line 2 can be used toprovide fuel to the high pressure compressor B and limit the amount ofnatural gas that must be purchased for use in feeding the high pressurecompressor B.

FIG. 2 is a schematic of a plant featuring a nitrogen rejection unit.Feed stock from a gas plant is fed through line 23 and open valve V1through the main heat exchanger J and into a flash separator L. Theflash separator will separate out the C2+ hydrocarbons from the feedstock and will return the C2+ hydrocarbons with the assistance of pump Oback through the main heat exchanger J where they are recovered throughline 21. The remainder of the feed stock which consists mainly ofmethane and nitrogen is fed from the top of the flash separator throughline 24 to the pressurized nitrogen rejection column M which will act toseparate the nitrogen from the natural gas and other components of thegas plant feed stock.

The nitrogen from the separation process in the pressurized nitrogenrejection column is collected from the top of this column through line22 and recovered after passing through the main heat exchanger J.

Gas for combustion purposes is supplied through line 20 and also passesthrough the main heat exchanger J and with the assistance of pump N isfed to the pressurized nitrogen rejection column.

A hydrocarbon heat pump compressor K will act to drive the reboiler Rand condenser P and will pass a cooler hydrocarbon mixture which ismainly methane from the compressor K through line 26 where it will passthrough the main heat exchanger J and pass cooling to the main heatexchanger J. The slightly warmer hydrocarbon mixture will continuethrough line 26 where it will provide some refrigeration to the reboilerR.

The bottoms from the pressurized nitrogen rejection column M, namely amixture of natural gas and other hydrocarbons is fed through line 31 tothe reboiler R. These hydrocarbons are fed to a subcooler Q through line27 and then through open valve V2 and line 28 to a condenser P. Thecondensed hydrocarbons are returned through line 32 to the pressurizednitrogen rejection column where they will provide cooler temperatures tothe column internals.

The warmer hydrocarbons that have been fed to the reboiler R will exitthe reboiler through line 30 and pass through the main heat exchanger Jwhere they will exchange heat and be fed through line 30 back to thehydrocarbon heat pump compressor K to provide some cooling to thecompressor before being fed back through the main heat exchanger Jthrough line 26 and cycled again through the reboiler R.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the invention,

Having thus described the invention, what we claim is:
 1. A method forenhanced oil recovery comprising the steps of (a) feeding nitrogen froma primary nitrogen supply into an oil field; (b) recovering oil and agas mixture comprising nitrogen, natural gas and C2+ hydrocarbons fromthe oil field; (c) separating the oil from the gas mixture (d) feedingthe gas mixture to a nitrogen rejection unit; and (d) recoveringnitrogen, natural gas and C2+ hydrocarbons.
 2. The method as claimed inclaim 1 further comprising feeding a supplemental nitrogen supply intothe oil field.
 3. The method as claimed in claim 1 wherein the primarynitrogen supply is from an air separation unit.
 4. The method as claimedin claim 2 wherein the supplemental nitrogen supply is from asupplemental air separation unit.
 5. The method as claimed in claim 1wherein the primary nitrogen supply is fed to a high pressurecompressor.
 6. The method as claimed in claim 1 wherein the highpressure compressor operates at a pressure up to 6700 pounds per squareinch.
 7. The method as claimed in claim 1 wherein the oil and the gasmixture are fed to an oil separation and sour gas removal unit, therebyseparating the recovered oil from the gas mixture.
 8. The method asclaimed in claim 1 wherein the recovered oil is fed to a storage system.9. The method as claimed in claim 1 wherein the nitrogen rejection unitwill separate the nitrogen from the natural gas and C2+ hydrocarbons.10. The method as claimed in claim 1 wherein the nitrogen rejection unitis operating at high pressures.
 11. The method as claimed in claim 10wherein the high pressure is 30 bar.
 12. The method as claimed in claim1 wherein the recovered nitrogen is fed to an intermediate pressurecompressor before being fed to the high pressure compressor.
 13. Themethod as claimed in claim 1 wherein a mixture of nitrogen and methaneis recovered from said nitrogen rejection unit and is fed to the highpressure compressor.
 14. The method as claimed in claim 9 furthercomprising feeding the C2+ hydrocarbons along with the nitrogen andmethane to the high pressure compressor.
 15. The method as claimed inclaim 1 wherein the recovered C2+ hydrocarbons are fed to a recycle orfractioning unit.
 16. A method for the recovery of natural gas from anenhanced oil recovery operation comprising the steps of (a) feeding aprimary nitrogen supply into an oil field; (b) recovering oil and a gasmixture comprising nitrogen, natural gas and C2+ hydrocarbons from theoil field; (c) separating the oil from the gas mixture; (d) feeding thegas mixture to a nitrogen rejection unit operating at elevatedpressures: and (e) separating natural gas from nitrogen and C2+hydrocarbons.
 17. The method as claimed in claim 16 further comprisingfeeding a supplemental nitrogen supply into the oil field.
 18. Themethod as claimed in claim 16 wherein the primary nitrogen supply isfrom an air separation unit.
 19. The method as claimed in claim 17wherein the supplemental nitrogen supply is from a supplemental airseparation unit.
 20. The method as claimed in claim 16 wherein theprimary nitrogen supply is fed to a high pressure compressor.
 21. Themethod as claimed in claim 16 wherein the high pressure compressoroperates at a pressure up to 6700 pounds per square inch.
 22. The methodas claimed in claim 16 wherein the oil and the gas mixture are fed to anoil separation and sour gas removal unit, thereby separating therecovered oil from the gas mixture.
 23. The method as claimed in claim16 wherein the recovered oil is fed to a storage system.
 24. The methodas claimed in claim 16 wherein the nitrogen rejection unit will separatethe natural gas from the nitrogen and C2+ hydrocarbons.
 25. The methodas claimed in claim 16 wherein the nitrogen rejection unit is operatingat elevated pressures.
 26. The method as claimed in claim 25 wherein thehigh pressure is 30 bar.
 27. The method as claimed in claim 16 whereinthe recovered nitrogen is fed to an intermediate pressure compressorbefore being fed to the high pressure compressor.
 28. The method asclaimed in claim 16 wherein the natural gas is recovered from saidnitrogen rejection unit and is fed to the high pressure compressor. 29.The method as claimed in claim 16 wherein the recovered C2+ hydrocarbonsare fed to a recycle or fractioning unit.
 30. A system for enhanced oilrecovery comprising a source of nitrogen, a supplemental source ofnitrogen, and a nitrogen rejection unit.
 31. The system as claimed inclaim 30 wherein the source of nitrogen is a primary air separationunit.
 32. The system as claimed in claim 30 wherein the supplementalsource of nitrogen is a supplemental air separation unit.
 33. The systemas claimed in claim 30 wherein the nitrogen rejection unit operates athigh pressure.
 34. The system as claimed in claim 30 wherein the highpressure is 30 bar.
 35. The system as claimed in claim 30 wherein thenitrogen rejection unit separates a gas mixture or nitrogen, natural gasand C2+ hydrocarbons.
 36. The system as claimed in claim 30 furthercomprising an intermediate pressure compressor.
 37. The system asclaimed in claim 30 further comprising a high pressure compressor.